The present invention relates to a high-performance sintered magnet formed from Rxe2x80x94Txe2x80x94B alloy powder produced by a reduction and diffusion method, and a method for producing such a sintered magnet.
Among rare earth permanent magnets, Rxe2x80x94Txe2x80x94B rare earth sintered magnets, wherein R is at least one rare earth element including Y, at least one of Nd, Dy and Pr being indispensable, and T is Fe or Fe and Co, are highly useful, high-performance magnets, much better in cost performance than Smxe2x80x94Co permanent magnets containing expensive Co and Sm. Accordingly, they are widely used in various magnet applications.
The Rxe2x80x94Txe2x80x94B rare earth alloy powder can be obtained by pulverizing alloys produced through melting, such as strip-cast alloys, alloys produced by high-frequency melting and casting, etc. Also, for instance a reduction and diffusion method (hereinafter referred to as xe2x80x9cR/D methodxe2x80x9d) provides less expensive Rxe2x80x94Txe2x80x94B alloy powder (hereinafter referred to as xe2x80x9cR/D powderxe2x80x9d). This Rxe2x80x94Txe2x80x94B alloy powder is produced by mixing rare earth element oxide powders, Fexe2x80x94Coxe2x80x94B alloy powder, Fe powder and a reducing agent (Ca) in proper formulations, heating the resultant mixture in an inert gas atmosphere to reduce the rare earth element oxides and diffuse the resultant rare earth metal into a metal phase of Fe, Co and B, thereby forming an Rxe2x80x94Txe2x80x94B alloy powder containing an R2T14B-type intermetallic compound as a main phase, removing reaction by-products such as CaO, etc. by washing, and then drying.
The R/D powder is less expensive than powder of alloys produced through melting, and thus more advantageous in reduction of the production cost of Rxe2x80x94Txe2x80x94B rare earth sintered magnets. However, the conventional R/D powder contains more inevitable impurities such as Ca, O, etc. than powder of alloys produced through melting. Therefore, Rxe2x80x94Txe2x80x94B rare earth sintered magnets formed from the R/D powder are poorer in squareness ratio of the demagnetization curve and more difficult in providing high-performance magnets than those formed from powders of alloys produced through melting. The poor squareness ratio means that desired magnetic flux cannot be obtained in permeance coefficients of magnetic circuits widely used in practical applications, leading to deterioration in thermal demagnetization. The squareness ratio is a value defined by Hk/iHc, wherein Hk is a value of H at a position at which 4xcfx80I is 0.9 Br (Br is a residual magnetic flux density) in the second quadrant of a graph of a 4xcfx80I-H curve, wherein 4xcfx80I represents the intensity of magnetization, and H represents the intensity of a magnetic field.
Japanese Patent Laid-Open No. 63-310905 discloses that products obtained by a reduction and diffusion reaction are washed with water containing 10xe2x88x92310xe2x88x922 g/L of an inhibitor (corrosion-suppressing agent), dewatered and then dried in vacuum to provide low-oxygen, low-Ca, Ndxe2x80x94Fexe2x80x94B permanent magnet alloy powder. However, when sintered magnets are obtained by subjecting the Ndxe2x80x94Fexe2x80x94B permanent magnet alloy powder (Ca content: 0.05-0.06 weight %) produced according to EXAMPLES of Japanese Patent Laid-Open No. 63-310905 to jet-milling, molding in a magnetic field, sintering in an Ar gas and a heat treatment, they contain more than 0.01 weight % of Ca, thereby being poor in squareness ratio and thermal stability.
Japanese Patent 2,766,681 discloses a method for producing rare earth-iron-boron alloy powder for sintered magnets comprising the steps of mixing rare earth oxide powders, iron-containing powder, B-containing powder and Ca, heating the resultant mixture at 900-1200xc2x0 C. in a non-oxidizing atmosphere, wet-treating the reaction product, heating it at 600-1100xc2x0 C., and finely pulverizing the resultant alloy powder to an average particle size of 1-10 xcexcm. In EXAMPLES of Japanese Patent 2,766,681, the R/D reaction product is washed with water, dried in vacuum, heat-treated in vacuum under the conditions shown in Table 1 below, cooled, finely pulverized, and then molded without a magnetic field, to provide a green body having improved bending strength. However, Japanese Patent 2,766,681 neither teaches the correlation between the heat treatment in vacuum in Table 1 and the amount of Ca remaining in the R/D powder at all, nor discloses that a combination of Ca removal by the heat treatment in vacuum of the R/D powder and Ca removal by the sintering in vacuum of the green body drastically reduces a Ca content in the Rxe2x80x94Txe2x80x94B rare earth sintered magnets, thereby remarkably improving the squareness ratio of the sintered magnets.
Accordingly, an object of the present invention is to provide an Rxe2x80x94Txe2x80x94B rare earth sintered magnet formed from Rxe2x80x94Txe2x80x94B rare earth alloy powder produced by a reduction and diffusion method, and a method for producing such an Rxe2x80x94Txe2x80x94B rare earth sintered magnet.
The method for producing an Rxe2x80x94Txe2x80x94B rare earth sintered magnet containing an R2T14B-type intermetallic compound as a main phase and thus having improved squareness ratio according to the present invention comprises carrying out a reduction and diffusion method comprising the steps of (a) mixing oxide powder of at least one rare earth element R, wherein R is at least one rare earth element including Y, at least one of Nd, Dy and Pr being indispensable, T-containing powder, wherein T is Fe or Fe and Co, B-containing powder, and at least one reducing agent selected from the group consisting of Ca, Mg and hydrides thereof, (b) heating the resultant mixture at 900-1350xc2x0 C. in a non-oxidizing atmosphere, (c) removing reaction by-products from the resultant reaction product by washing, and (d) carrying out a heat treatment for Ca removal by heating the resultant Rxe2x80x94Txe2x80x94B rare earth alloy powder at 900-1200xc2x0 C. in vacuum at 1 Torr or less, followed by pulverization of the resultant alloy powder bulk, molding, sintering in vacuum, heat treatment, and surface treatment. The alloy powder bulk obtained by the heat treatment for Ca removal is preferably pulverized after removal of its surface layer.
The Rxe2x80x94Txe2x80x94B rare earth sintered magnet having improved squareness ratio according to the present invention contains as a main phase an R2T14B-type intermetallic compound, wherein R is at least one rare earth element including Y, at least one of Nd, Dy and Pr being indispensable, and T is Fe or Fe and Co, the amount of Ca contained as an inevitable impurity being 0.01 weight % or less, and c-axis directions of core portions of the main-phase crystal grain particles being deviated by 5xc2x0 or more from those of surface layer portions of the main-phase crystal grain particles. In the metal structure of the Rxe2x80x94Txe2x80x94B rare earth sintered magnet, the number of the main-phase crystal grain particles having surface layer portions is preferably 50% or less of the total number of the main-phase crystal grain particles.
The composition of the Rxe2x80x94Txe2x80x94B rare earth sintered magnet preferably comprises as main components 27-34 weight % of R, and 0.5-2 weight % of B, the balance being substantially T, and the amounts of oxygen and carbon contained as inevitable impurities being 0.6 weight % or less and 0.1 weight % or less, respectively. The Rxe2x80x94Txe2x80x94B rare earth sintered magnet preferably has a squareness ratio of 95.0% or more at room temperature.